Large aircraft, for example of the wide-body type, typically employ multi-wheel landing gear or landing gear trucks. To prevent tire scrubbing, and hence tire wear, during taxiing, such multi-wheel landing gear typically are provided with some form of steering. For example, the multi-wheel landing gear may include multiple pairs of wheels mounted on respective axles carried by a beam (commonly referred to as a bogie beam), and either the rear axle, or both forward and rear axles, on the truck is steerable. Each steerable axle may be operated by one or two hydraulic actuators typically controlled by the nose gear steering system. U.S. Pat. Nos. 5,595,359 and 5,613,651 disclose steerable landing gear wherein each steerable axle is driven by a single center-locked actuator.
During landing and take-off, each steerable axle preferably is locked against turning. For this purpose, the actuator may be internally locked in its center stroke position as described in U.S. Pat. No. 5,595,359. In U.S. Pat. No. 5,242,131, such function was provided by a separate external hydraulic plunger-lock mechanism.
The external locking mechanism described in U.S. Pat. No. 5,242,131 has several drawbacks. Such mechanism included a plunger pin movable by hydraulic pressure into an aperture in a tongue provided on the steerable axle. Consequently, hydraulic pressure is required to engage the locking mechanism. Thus, if hydraulic pressure is lost, the locking mechanism cannot be engaged. In the event of a loss of hydraulic pressure after the locking mechanism has been engaged, the plunger pin could possibly become dislodged from the aperture and no longer hold the steerable axle against swivelling.